Elevated serum phosphorus (P) and related abnormalities in P homeostasis due to progressive loss of kidney function are primary driving forces behind cardiovascular dysfunction and mortality in chronic kidney disease (CKD) patients. In addition, elevated serum P is associated with increased mortality in the healthy general population. Intestinal P absorption is a critical, yet understudied component in P metabolism. Elucidating factors that influence and regulate P absorption is essential for developing strategies to control abnormal P metabolism and associated consequences in CKD patients, and in the general population. The principle investigator (PI)'s long-term career goal is to develop an independent clinical and translational research program focused on determining the factors that influence and regulate intestinal P absorption in healthy and CKD populations. To achieve this, the PI will conduct a series of clinical studies in healthy subjects and patients with CKD, supported by complementary experiments in animal models of CKD. This fits well within the mission of the NIDDK to improve health and quality of life through support of research and research training in kidney disease and endocrine and metabolic diseases. The goals of the specific aims of this project are: 1) to determine intestinal P absorption efficiency in patients wth early stages of CKD in comparison to healthy adults, how P absorption is affected when dietary P is restricted, and to determine P bioavailability from organic and inorganic dietary sources; 2) to determine P absorption efficiency and response to dietary P intake on P absorption in a mechanistic study using the Cy/+ rat model of progressive CKD-MBD, and determine the role of localization of the intestinal P transporter NaPi-2b on P absorption; and 3) to determine total-body P balance in normal children and the effects of dietary Ca intake, demographic, and biochemical factors on P balance leading to future studies in children with CKD. A team of mentors with complementary expertise relevant to this project will oversee the PI's research training in P absorption and metabolism, as well as career development guidance during the mentored award period. The PI will partake in educational experiences in bone and mineral metabolism, chronic kidney disease, clinical research, and responsible conduct of research. Purdue University and Indiana University School of Medicine, along with the Indiana Clinical Research Center, provide excellent environments in terms of research and intellectual resources for clinical and basic studies in mineral metabolism.